Add client_id field to canvas model for renaming : part 1/3
part 1/3 in renaming 'key' field to 'client_id' per https://openedx.atlassian.net/browse/ENT-3290
part 1/3 in renaming 'key' field to 'client_id' per https://openedx.atlassian.net/browse/ENT-3290